The long-term goal of this proposal is to understand the fundamental biophysical properties that link the human MutS homologues (MSH) and MutL homologues (MLH/PMS) to their essential role(s) in signaling mismatch repair (MMR) and/or damage-induced apoptosis. During the last granting period we made considerable progress in our studies of the biochemistry, genetics, cell biology, and tumor biology of the human MMR system. Our studies have detailed the breadth of MSH recognition/activation as well as the distinct role(s) of the MLH/PMS proteins in these processes. Moreover, we appear to have discovered a common DNA signature that underpins the ubiquitous recognition properties of MSH proteins. In this two-year bridge application, we propose to focus on the ATP/ADP-dependent allosteric control within and between human MSH proteins that discriminate their function(s) in either MMR or damage-induced apoptosis. We also propose to examine the effect of novel well-defined human chromatin substrates on MSH and MLH/PMS function(s). These goals will be accomplished by the following Specific Aims: I.) Examine the essential conformational transition(s) that link hMSH2-hMSH6 and hMSH2-hMSH3 ATP-dependent function(s) to MMR initiation and/or Apoptotic signaling, and II.) To detail the effect of defined chromatin substrates on MSH function(s). These studies are designed to draw from the unique biophysical strengths of this laboratory toward a detailed understanding of MSH and MLH/PMS adenosine nucleotide function(s) and dysfunction(s) in MMR, apoptosis, and tumorigenesis. Insight into these processes should contribute to our understanding of ABC and GHKL NTPases as well as aid therapeutics, therapeutic sensitization, and chemoprevention protocols for HNPCC-related tumors.